Image sensor arrays, such as found in digital document scanners and digital cameras, typically comprise a linear array of photosites which raster scan a focused image, or an image bearing document, and convert the set of microscopic image areas viewed by each photosite to image signal charges. Following an integration period the image signal charges are amplified and transferred to a common output line or bus through successively actuated multiplexing transistors.
Currently there are two generally accepted basic technologies for creating such linear arrays of photosites: Charge-coupled devices, or CCD's, and CMOS. In CMOS, the photosensors are in the form of photodiodes, which output a charge in response to light impinging thereon. In the scanning process, bias and reset charges are applied in a predetermined time sequence during each scan cycle. Certain prior art patents, such as U.S. Pat. No. 5,081,536 assigned to the assignee hereof, disclose two-stage transfer circuits for transferring image signal charges from the photosites in CMOS image sensors.
In designing photosensitive devices using photodiodes, it is desirable to use signals from the photodiodes which are created toward the middle portion of the photodiodes'response, where the response function is highly linear. In other words, light-responsive signals from the lower portion of a photodiode's response tend not to be linear, and thus unreliable as a reflection of the amount of light integrated by the photodiode at a particular time. In order to exploit the more linear middle portion of a photodiode's response, one technique, which is used in the patents incorporated by reference above, is to inject a predetermined bias charge, or “fat zero,” onto the photodiode with each cycle of operation wherein light energy is integrated as a charge on the photodiode and then transferred through a transfer circuit. The fat zero bias in effect “primes the pump” of charge within the photodiode so that the nonlinear portions of the photodiode response are not used.
In practical applications of photosensitive devices using fat zero bias, two key sources of noise, which can affect the integrity of the output image signals, are “fixed pattern noise” and “thermal noise.” The first of these types of noise relates to the fact that, within any device, individual photodiodes and sets of circuitry associated with the various photodiodes will have some variation in performance, and the variation in performance among the different sets of circuitry will result in a fixed pattern of noise effecting the signals, resulting in a consistent pattern of distortions in the output signals, depending on which specific set of circuitry a particular subset of the video signals passes through. Thermal noise is created by the fact that the output of a particular set of circuitry is likely to change over time, due to the random thermal movement of electrons in conductors.
It is an object of the present invention to overcome these customary sources of noise by sampling actual values of charge placed on photodiodes during the course of operation of an apparatus, and then using these actual sampled values to correct subsequent video signals.